With an increasing capacity of a wavelength multiplexing optical transmission system, the influence of a communication shutoff due to a failure of an optical transmission line is growing. Thus, to improve the reliability of an optical transmission system, backup redundancy of an optical transmission line and switching to a backup system at the time of a failure of the active system is effective.
On the other hand, as for a long distance optical transmission system with optical repeaters, there is a danger that transitional power changes of signal light rays at the switching between the optical transmission lines can bring about an optical surge in an optical amplifier of a downstream optical repeater, thereby damaging its downstream optical receiver because of excessive power. As a conventional optical transmission line switching apparatus, Patent Document 1, for example, discloses an optical receiver that switches the received light rays to a backup route between the two routes of an active system and a backup system if the power of the received light rays from the active route reduces. In addition, Patent Document 2 discloses a method of avoiding an optical surge occurring in an optical amplifier at the time of switching between the optical transmission lines.